arcobjects - How to create a transparent buffer of a polyline?

I use vb.net arcobjects create a buffer around a polyline feature once it is selected. The problem is the buffer has a obvious colour. Since I have other functions on the selected feature, I cannot clear the buffer because user may still need the feature selected.

When I select another feature, the new buffer will created and old buffer cleared. But there always a buffer which makes the polyline there looks different from other features in the same layer.

So I am thinking whether can make the buffer transparent so that it wouldn't affect the overview of the map.

My code is from an ESRI web sample:

 Public Sub CreateGraphicBuffersAroundSelectedFeatures(ByVal activeView As ESRI.ArcGIS.Carto.IActiveView, ByVal distance As System.Double)

    'parameter check
    If activeView Is Nothing OrElse distance < 0 Then
        Return
    End If


    Dim map As ESRI.ArcGIS.Carto.IMap = activeView.FocusMap
    ' Clear any previous buffers from the screen
    Dim graphicsContainer As ESRI.ArcGIS.Carto.IGraphicsContainer = CType(map, ESRI.ArcGIS.Carto.IGraphicsContainer) ' Explicit Cast
    graphicsContainer.DeleteAllElements()

    ' Verify there is a feature(s) selected
    If map.SelectionCount = 0 Then
        Return
    End If


    ' Reset to the first selected feature
    Dim enumFeature As ESRI.ArcGIS.Geodatabase.IEnumFeature = CType(map.FeatureSelection, ESRI.ArcGIS.Geodatabase.IEnumFeature) ' Explicit Cast
    enumFeature.Reset()
    Dim feature As ESRI.ArcGIS.Geodatabase.IFeature = enumFeature.Next()

    ' Buffer all the selected features by the buffer distance and create a new polygon element from each result
    Dim topologicalOperator As ESRI.ArcGIS.Geometry.ITopologicalOperator
    Dim element As ESRI.ArcGIS.Carto.IElement
    Do While Not (feature Is Nothing)

        topologicalOperator = CType(feature.Shape, ESRI.ArcGIS.Geometry.ITopologicalOperator) ' Explicit Cast
        element = New ESRI.ArcGIS.Carto.PolygonElementClass()
        element.Geometry = topologicalOperator.Buffer(distance)
        graphicsContainer.AddElement(element, 0)
        feature = enumFeature.Next()
    Loop

    activeView.PartialRefresh(ESRI.ArcGIS.Carto.esriViewDrawPhase.esriViewGraphics, Nothing, Nothing)

End Sub

I am using vb.net arcobjects and arcmap 10.2. Any advice on it?

Answer

The following VBA code grabs the first polygon graphic on the Map and sets the transparency. The problem is that graphics don't appear to support true transparency. In the Help for IColor.Transparency is states:

...For graphic elements, 0 for transparent and 255 for opaque are the only supported values...

Public Sub MakeTransparent()
    ' Get map document
    Dim pMXDoc As IMxDocument

    Set pMXDoc = ThisDocument

    ' Get Map
    Dim pMap As IMap
    Set pMap = pMXDoc.FocusMap

    ' Get Graphics Container for map
    Dim pGC As IGraphicsContainer
    Set pGC = pMap
    pGC.Reset


    ' Get first graphic, assumed to be polygon
    Dim pElement As IElement
    Set pElement = pGC.Next

    ' Get symbol of graphic and its colour
    Dim pFillShapeElement As IFillShapeElement
    Set pFillShapeElement = pElement
    Dim pFillSymbol As IFillSymbol
    Set pFillSymbol = pFillShapeElement.Symbol

    Dim pColour As IColor
    Set pColour = pFillSymbol.Color

    ' Set transparency
    pColour.Transparency = 0 ' Only 0 or 255 is support
    pFillSymbol.Color = pColour
    pFillShapeElement.Symbol = pFillSymbol

    ' Refresh map
    pMXDoc.ActiveView.Refresh

End Sub